The Stirling Engine was named by Dr. Rolf. J. Meijer who at that time
was a project manager with Philips of Holland. Philips was struggling
with creating a new name to call the 'Air' engine when there was no air
inside the engine. This is because in an Air engine, the air inside
the engine is called the 'working gas'. If you change the 'working
gas' to a gas like Helium or Hydrogen, then it can no longer be called
an 'Air' engine. The name Stirling Engine was chosen in honor of
the inventor of the regenerator (economizer) and the engine that demonstrated
its use.

The Stirling Engine's most basic configuration consists of two pistons
each in its own cylinder. (Sometimes it is easier to envision these
two cylinders as one long tube with the piston heads facing each other
inside the tube (see the figure below)). Note that between these two pistons
heads are the heater, cooler and regenerator. The regenerator (usually
a block of woven wire) is in the center of this tube and the heater is
between the regenerator and one piston (in red) while the cooler is between
the regenerator and the other piston (in Blue). The volume attached to
the 'heater' is the 'expansion space' where the hot gas pushes against
the 'expansion piston'. The volume attached to the 'cooler' is the
'compression space'.
The regenerator is where the excess heat of the gas is stored
in the regenerator matrix on the way to the compression space from
the expansion space and then the heat is recovered on the way back from
the compression space to the expansion space.

Graphic courtesy of Dr. Israel Urieli of Ohio University.

Stirling Engine operation can be explained in a somewhat non technical way that
applies to many but not to all engines that may be called Stirling Engines.

We begin with the heater space and cooler space at their appropriate temperatures.
The working gas trapped between the two piston heads is pushed by the
Compression Piston through the regenerator where it is heated by the energy
stored in the regenerator to the maximum temperature present in the regenerator
into the Heater volume where the gas expands due to increased temperature
into the Expansion Space. This results in an increased pressure pushing on the Expansion
Piston so that it moves away from the regenerator pushing on a mechanism
which changes the linear movement of the piston to a rotary motion.
This continues until all the gas that will expand has been pushed into
the heater area and expanded. The mechanism continues to push the Compression
Piston further toward the Regenerator pushing all the gas out of the Compression
Space into the gas circuit (heater, cooler, regenerator).

Once the expansion piston moves to its extreme position, the mechanism, to which
both pistons are connected (but 90 rotary degrees apart), now begins to move the
Expansion piston back the other way pushing the hot gas back through the Heater and
then to the Regenerator and finally into the Cooler where it begins to Cool and
contract (the pressure starts to drop). The Compression Piston is also moving away from
the regenerator while the Expansion piston comes toward the regenerator
moving the gas through the regenerator into the compression space without
compressing the gas.

The linkage continues to move the pistons until the Compression Piston
is at its extreme and the Expansion piston is all the way forward.
At this point the mechanical arrangement moves the pistons together but
because of the way the piston moves up and down in the cylinder but the
mechanism is moving in a circle, the Expansion Piston does not move
very far but the Compression Piston moves toward the regenerator actually
compressing the gas and begining to push the gas through the regenerator.
(That is why it is called the Compression Piston.)

This brings us again to the first line of this explanation to complete
the cycle and begin again.

A Stirling "Air" Engine is a mechanical device which operates on a closed
regenerative thermodynamic cycle with cyclic compression and expansion
of the working fluid (air) at different temperature levels. The flow of
the working fluid is controlled by changes in the volume of the
hot and cold spaces, eliminating the need for valves. The Stirling
Engine process (cycle) is reversible, meaning that an input of heat energy (burning fuel,
for example) will produce an output of mechanical energy, and an input
of mechanical energy (electric motor, etc.) will produce an output of heat
energy. In this manner, the Stirling Engine can be used as a heat pump
in much the same way as traditional refrigeration units, only using something other
than the environmentally harmful refrigerants.

The most basic engine consists of a set of pistons, heat exchangers,
and a device called a 'regenerator'. The engine is filled with a working
fluid (gas) which is commonly Air, but some more advanced engines may use
Nitrogen, Helium or Hydrogen. The pistons are arranged such that they create
both a change in volume of the working fluid and create a net flow of the
fluid through the heat exchangers. In this manner, heat is absorbed from
an external source in the 'hot' end, creating mechanical energy,
and rejected in the 'cold' end to the environment.

In a Stirling engine, the working fluid is completely contained inside
the engine at all times, meaning the cycle is closed, As opposed
to a typical gasoline engine, which takes in 'fresh air' for each new cycle.
This enables a Stirling Engine to operate cleanly and quietly
as there are no combustion products coming into contact with any of the
engine's working components and no release of high-pressure gasses.

An important feature in Stirling Engines is the regenerator.
On the most basic level, a regenerator is a device that absorbs heat from
the working fluid as it enters the 'hot' end, and re-heats the fluid as
it enters the 'cold' end. This internal recycling of energy allows for
much higher efficiencies, and better performance overall. The regenerator
is such a critical component that most Stirling Engines cannot operate
efficiently without one!

Stirling Engineering ( Deeper
understanding)This link is a much deeper look into the theory. Click
Here for a look at the Detailed Theory of Operation.This information is from Dr. Israel Urieli of Ohio University.
Caution: Contains calculus, partial differential equations and thus requires
a knowledge of the calculus. Also contains source code modules for a second
order simulator (In 'C').

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